Switching DC/DC power converters designed to be efficient at light loads typically use a hysteretic control technique, sometimes referred to as Burst Mode or Pulse Frequency Mode, to regulate output voltage. In such operation, the converter operates at a fixed power level, usually by regulating a peak inductor current, until the output achieves the desired voltage. The converter then goes into a “sleep” (inactive) operational mode, drawing minimal quiescent current from the power source. During the inactive mode, the load current is supplied only by an output filter capacitor. When the output voltage has dropped by a small amount, typically one to two percent, the converter comes out of the sleep mode and resumes active operation to bring the output voltage back up to the desired value. The cycle of alternating periods of active and inactive operational modes repeats, maintaining the output voltage within the specified hysteretic limits. The time duration of the active, or “wake”, time and sleep time varies with the amount of output capacitance and the amount of hysteresis chosen. The percentage of time spent awake or asleep, i.e., duty cycle, varies with load. At its maximum load capability, the converter stays awake one hundred percent of the time.
Typical converter architecture may comprise step-up (boost), step-down (buck) or step-down/step-up (buck-boost) designs. A known “four-switch” buck-boost converter is described, for example, in an October 2001 datasheet for the LTC3440 “Micro-power Synchronous Buck-Boost DC/DC Converter” integrated circuit manufactured by Linear Technology Corporation. During the active mode, an inductor is switched among various circuit configurations to apply charge to the output capacitor. In active burst operation, inductor current traditionally is controlled to vary, cyclically, between fixed upper and lower limits, commonly called peak and valley levels, respectively.
An advantage of burst mode converter operation, as compared to fixed frequency pulse width modulated switching operation, is high efficiency at light loads, because the percentage of time that the converter is asleep increases as the load current diminishes. If the quiescent current of the hysteretic converter can be made very small, typically tens of micro-amps, while in the sleep mode, high efficiency can be maintained until the load current drops to as little as one hundred micro-amps or less. This operation is advantageous for battery powered applications that spend considerable time in an idle state that requires little power.
A disadvantage of the burst mode operation is that the maximum output power that can be delivered is limited by the peak inductor current, which is fixed regardless of load. If the peak inductor current is raised to increase power capability, the converter's conduction losses are increased, which lowers efficiency across the entire load range. Therefore, in a hysteretic converter, a fixed peak inductor current value is chosen as a compromise between efficiency and maximum power capability. If maximum power capability is increased, the increased peak inductor current results in lower efficiency at light loads. Difficulty in sensing load current while maintaining hysteretic operation, presents challenges in departing from fixed peak inductor current operation.